See my blog post on how to persist n8n custom and community nodes in your docker setups.
Post V1.0 there are a few changes to permissions so pay close attention to where things are getting installed. n8n now runs as the node user.
The easiest option is to use the example in 'bwill-nodes-simple' folder -
FROM n8nio/n8n:latest
# Post v1.0:
RUN mkdir ~/.n8n/nodes && cd ~/.n8n/nodes && npm install n8n-nodes-clicksend && npm install .....
See migration checklist here: https://docs.n8n.io/1-0-migration-checklist/